In order to operate the reverse osmosis membrane for a long period of time in a stable manner, an appropriate pretreatment such as sterilization of the water to be treated or removal of turbid components and a periodical washing of the membrane are unavoidable. That is because, when fouling components are sedimented on the surface of the osmosis membrane or slime by the growth of microbes is adhered thereon, significant reduction of the property is resulted. Examples of the pretreatment include removal of the scales and the fouling-causing substances by adjustment of pH or by a membrane treatment. Examples of the membrane washing include the use of solution of alkaline chemicals and solution of chlorine-type chemicals or, particularly, sodium hypochlorite.
As to the reverse osmosis membrane which has been commercially used up to now, cellulose triacetate (CTA, Patent Document 1) and aromatic polyamide (PAm, Patent Document 2) are main ones. Although CTA membrane is excellent in its resistance to chlorine as compared with PAm membrane and is particularly excellent in terms of the washing with sodium hypochlorite and the recovery of membrane property, it is weak in terms of resistance to alkali and has a disadvantage that the use of solution of alkali-type chemicals is limited. On the contrary, although PAm membrane is excellent in terms of resistance to alkali as compared with CTA membrane, its resistance to chlorine is very low and it has a disadvantage that the use of solution of chlorine-type chemicals is limited.
To improve those disadvantages, Patent Document 3 proposes a separating membrane using a polymer having a sulfonated poly(aryl ether sulfone) structure. The membrane prepared as such is improved in its resistance to chemicals but, on the other hand, there is a disadvantage that its water permeability is bad and it is not suitable for practical use. For solving this problem, Patent Document 4 attempts that both water permeability/salt rejection rate and chemical durability are achieved by means of thinly coating the sulfonated poly(aryl ether sulfone) on a compounded membrane of a PAm type. According to such a method however, there are problems that it is risky for maintaining the resistance to chemicals for a long period due to its thin coating layer and also that cost becomes high due to an increased complication of the membrane-manufacturing steps.
As mentioned hereinabove, the conventional reverse osmosis membranes have a problem in any of salt rejection rate, water permeability and resistance to chemicals and it is the current status that the product where all of those properties are actualized in a high level has not been available yet.